European patent application No. 0 651 409 A1 describes a mechanism of this type. In that embodiment, all of the teeth of the small toothed wheel and the first tooth following, in the predetermined direction, the gap formed by the discontinuity in the teeth of the large toothed wheel are shaped in such a manner that their flanks converge on respective single common edges. The purpose of that measure is to prevent the toothed wheels jamming. Nevertheless, it turns out that the friction between an edge of the small toothed wheel against the leading flank of the first tooth of the large tooth wheel immediately after the gap gives rise to a loss of power of greater or lesser extent due to friction. Such losses of power depend on the position of the small toothed wheel, and as a result they give rise to variations in the time taken to engage the switchgear as well as to non-negligible wear on the first tooth of the large toothed wheel immediately following the gap. If the mechanism is used for controlling a switch that is designed to be engaged to reduce surges on a network synchronously with the voltage of the network, then such variations appear in random manner and are not acceptable.
European patent No. 0 294 561 A2 describes a drive mechanism of similar type, coupled to a disconnector and having a first tooth of the large toothed wheel that is situated immediately following the gap which is radially retractable against the urging of a compression spring. If this tooth comes into abutment against the top of a tooth of the small toothed wheel at the beginning of the engagement process, then it retracts radially, compressing the spring with which it is associated. As a result, this tooth can slide over the edge of the corresponding tooth of the small toothed wheel without giving rise to jamming. After it has gone past the edge, it engages in the space following the tooth of the small toothed wheel. Nevertheless, there exists a large risk of the retractable tooth continuing to drive the large toothed wheel by friction, in spite of it being progressively retracted into the housing which contains the spring. Under such circumstances, the second tooth following the gap formed on the large toothed wheel, which second tooth is not retractable, can itself come into abutment against the edge of a tooth of the small toothed wheel and jam the entire mechanism. This probability is high, since the small toothed wheel is decoupled from its drive by a freewheel coupling and therefore can turn very easily.